(1) Field of the Invention
The present invention relates to a process for producing a grain-oriented thin electrical steel sheet having a high magnetic flux density and excellent product magnetic characteristics, by a one-stage cold-rolling method. More particularly, the present invention relates to a process for producing a thin steel sheet having a thickness of up to 0.17 mm.
(2) Description of the Prior Art
A grain-oriented electrical steel sheet is used mainly as a soft material for a magnetic core of a transformer or other electric appliances, and must have excellent magnetic characteristics such as exciting and core loss characteristics.
To obtain an electrical steel sheet having such excellent characteristics, the &lt;100&gt; axis, i.e., the easy magnetization axis, must have a good agreement with the rolling direction. Note, the sheet thickness, crystal grain size, inherent resistance, and surface coating have a great influence on the magnetic characteristics.
The directionality of an electrical steel sheet is greatly improved by a one-stage cold-rolling process conducted under a high pressure and in the presence of an inhibitor such as AlN or MnS; currently this process can produce a steel sheet having a magnetic flux density corresponding to about 96% of the theoretical value thereof.
To cope with recent increases in energy costs, transformer manufacturers are attaching much importance to the use of a small-core-loss magnetic material as a material for an energy-saving transformer. Accordingly, an amorphous alloy or a high-Si alloy such as a 6.5%-Si alloy has been developed as the small-core-loss magnetic material, but this alloy is unsatisfactory as a material for a transformer from the viewpoint of cost and processability.
The core loss of an electromagnetic steel sheet is greatly influenced by not only the Si content but also the sheet thickness, and it is known that if the sheet thickness is reduced by chemical polishing or the like, the core loss is reduced.
In Japanese Unexamined Patent Publication No. 58-217630, the present invention proposed a process in which a silicon steel slab containing acid-soluble Al, N, and Sn is used as the starting material and a unidirectionally grain-oriented thin electrical steel sheet having a high magnetic flux density is produced by the high-pressure one-stage cold-rolling method, including the annealing of a hot-rolled steel sheet. This process enabled a grain-oriented thin electrical steel sheet having an excellent core loss and a high magnetic flux density, especially a thin steel sheet having a thickness reduced to 0.225 mm, to be manufactured at a low cost and on an industrial scale, and thus contributed to the eagerly desired saving of energy through a reduction of the core loss in transformers manufactured by using this steel sheet.
Nevertheless, current demands for saving energy have increased, and it has become necessary to further enhance the performance of a grain-oriented electrical steel sheet as a material for a transformer. Namely, it has become necessary to establish a process capable of producing a grain-oriented thin electrical steel sheet having a high magnetic flux density and a thickness of up to 0.175 mm, and having a much smaller core loss than that of the steel sheet having a thickness of 0.225 mm.
Note, according to the process disclosed in Japanese Unexamined Patent Publication No. 58-217630, a steel sheet having a thickness of 0.175 mm or 0.150 mm can be produced, but as shown in Tables 8 and 11 of this patent publication, in the case of a steel sheet having a thickness of up to 0.175 mm, the secondary recrystallization is not satisfactory, and when the process is carried out on an industrial scale, the yield is low and the level of the magnetic characteristics and the stability is not sufficiently high.